OPTICAL PROPERTIES OF NUCLEIC ACIDS 537 



For oriented sheets 



I>sheet I = Sin^ ^/COS^ e 

 I>sl,eet II = Sin2 e 



Sheet model type I is for the case in which the normal to the planes of the 

 absorbing groups makes an angle 6 with the direction of shear and lies in a 

 plane normal to the direction of the incident radiation. In the type II model 

 the normal to the absorbing planes lies at an angle 6 to the direction of 

 shear and in the plane of incidence of the radiation. 



By using these equations some indication of the mean orientation of the 

 absorbing groups can be obtained. Since observations on crystals with 

 favorably disposed absorbing groups indicate that absorption is not van- 

 ishingly small when the electric vector is perpendicular to the ring plane, 

 these simple relationships have to be modified if it is required to estimate 

 d from very large or very small dichroic ratios. 



4. DicHROisM OF Simple Compounds 



A much-quoted demonstration of the dichroism of the benzene chromophore is due 

 to Scheibe, Hartwig, and Miiller using hexamethylbenzene.ii'This compound crystal- 

 lizes with all the planar molecules parallel to one another and can be obtained as 

 crystalline sheets or sublimed crystalline films in which the planes of the benzene 

 groups are roughly normal to the specimen plane. Such a preparation has a dichroic 

 ratio of ca. 10 over most of the absorption band with X^ax. ca. 270 niju (corresponding 

 to the well-known "forbidden" benzene band at ca. 255 van). Scheibe et al. also find 

 that the wavelengths of the vibrational fine-structure bands are slightly different 

 for the electric vector parallel and perpendicular to the ring planes. 



Craig and Lyons"^ have extended these observations to the 230-mM band, which 

 is also polarized in the molecular plane, the dichroic ratio for this band being of the 

 same order. This result, which is applicable by analogy to the benzene "allowed" 

 system of ca. 200 m^, is of great importance for current theoretical arguments about 

 the assignment of the benzene 200-mM system and the nature of the molecular orbitals 

 involved. 



Dichroism studies on the particular pyrimidines and purines found in nucleic acids 

 have not yet been reported, probably because of difficulty in obtaining suitable 

 crystal specimens and the lack of X-ray structure data. Seeds'o^.ne.ne has studied 

 2-hydroxy-4,6-dimethylpyrimidine (I) for which favorable X-ray data are available. 



Me Me Me 



y\ ./\ .^ 



N 

 ill 



N N 



HO^N/Me HoN'^j^t/C1 C^n/^® 



I II III 



"3 G. Scheibe, St. Hartwig, R. Muller, Z. Elektrochem. 49, 372 (1943). 

 "" D. P. Craig and L. E. Lyons, Nahire 169, 1102 (1952). 

 "6 W. E. Seeds, Ph.D. Thesis, London University, 1951. 

 "6 W. E. Seeds, Discussions Faraday Soc. No. 9, 394 (1950). 



